A friend just returned from a week's break to Asia flying SQ via SIN. Offered the bid to upgrade to PEY both ways and they submited bids for $220 (there were one to two clicks above the minimum allowed of $200 each way).
The MEL-SIN flight was fairly empty in economy - bid to upgrade was unsuccessful. But they were very happy upper deck in one of the pairs of seats by the window.
The return SIN-MEL was packed in economy, the upgrade was successful. PEY was empty.
So it seems there might be some link here... if economy is full, rather than op-up they will accept low bids. Dunno if a higher bid will be successful for flights that are empty in economy. I guess they might be.
For meals, the menus are the same physical size, the differences being a choice of three mains in PEY rather than two in economy, and the addition of champagne in the drinks list. The second meal service (refreshment) was identical for PEY and Y.
Of course you can also book the cook in PEY.